The present invention relates to a structure for mounting a screen grid of a corona charger which is used to charge the surface of a photoconductive member in an electrophotographic process.
Imaging devices (so-called printers) for providing a hard copy by printing or drawing output data from a computer, word processor, facsimile and the like on a recording paper by employing a so-called electrophotographic process. In the electrophotographic process, a photoconductive material on the surface of a photoconductive member is uniformly charged, a latent image is formed onto a portion of the photoconductive member by photoconductive material from which electric charges are removed by exposure, this latent image is developed by depositing toner thereon (made to a toner image), then the toner image is transferred to a recording paper and further fixed thereto by a fixing unit.
Such imaging device usually charges the photoconductive material on the surface of the photoconductive member by a corona charger provided in close proximity to the photoconductive member.
The corona charger includes charge wires and a shield plate surrounding the charge wires and opened toward the photoconductive member side. When a high voltage is imposed on the charge wires, corona discharge is directed toward the photoconductive member and ambient air which is ionized by the corona discharge is moved against the photoconductive material, thereby charging it.
To create a good image, a constant electric potential must be charged on the surface of the photoconductive member. For this purpose, conventionally, a screen electrode plate formed to a grid shape or the like (i.e., a so-called screen grid) is provided to the opening of the shield plate of the corona charger confronting the photoconductive member, and a current flowing to the photoconductive material is controlled by a voltage imposed on the screen grid to thereby stabilize the electric potential on the surface of the photoconductive material.
The screen grid is grounded through a Zener diode or varister so that a constant voltage is imposed on the screen grid.
In the above arrangement, since the screen grid must have pinpoint positional accuracy with respect to the photoconductive member to charge a constant electric potential on the surface of the photoconductive member with a high accuracy, the positioning and mounting of the screen grid is very tedious and time-consuming.